The present invention relates to a composite material of the carbon-carbon type made stainless up to a temperature of 1800.degree. C., under a low air pressure, as well as to its production process.
This composite material is more particularly intended for use as a high performance heat protection for space vehicles (shuttles or aircraft) having to resist heating caused by the friction of the air when they reenter the atmosphere at high speed.
However, the invention is also applicable to other industrial fields requiring the use of structures able to resist high mechanical stresses under temperature above 1100.degree. C. in a corrosive medium. This is particularly the case with improved efficiency turbine engines operating at high temperatures (between 1300.degree. and 1400.degree. C.) and certain industrial heat recuperators.
The major problem of carbon-carbon type composite materials is their oxidation in air at high temperature causing the carbon to be converted into CO or CO.sub.2, so that the composite material is deteriorated and even destroyed. To prevent said oxidation, different processes have already been considered with regards to the protection of the carbon-containing materials and based on the use of a silicon carbide (SiC) coating formed on the outer surface of the composite material parts.
This outer SiC coating can be obtained by conversion into a silicide or siliciding the carbon of the outer portion of the material. Siliciding is carried out by pack cementation, as described in U.S. Pat. No. 3,095,316, or by chemical vapor phase deposition (CVD/CVPD). The deposition of SiC by CVD can be carried out by depositing a silicon coating on the outer portion of the material and then melting the silicon in order to ensure its diffusion into the material and its reaction with the carbon of the latter to form the SiC, as described in U.S. Pat. Nos. 3,406,044 and 3,925,577.
This outer SiC coating can also be obtained directly by SiC deposition based on the cracking of chlorosilane vapors, either alone or combined with hydrogen or hydrocarbons, as described by the article by S. Audisto in Actualie Chimique, September 1978, pp 25-33.
Other methods combine the siliciding of the surface carbon of the composite material with a chemical vapor phase deposition (cf. U.S. Pat. Nos. 3,406,044, 4,425,907 and 4,476,178.
All the aforementioned methods for producing a SiC coating on carbon-carbon type composite materials lead to the obtaining of a cracked coating as a result of the variation in the expansion coefficients between the carbon and the silicon carbide. Moreover, in order to obviate this disadvantage, with the outer SiC coating has been combined a silica coating for sealing the cracks of the SiC coating (cf. "Weight uncertainty analysis for space shuttle reinforced carbon-carbon" SAWE Paper 1265-37th Annula Conference May 8-10, 1978).
In order to improve the sealing of the cracks of the SiC coating, consideration has also been given to the user of a coating based on SiC powder and a sodium silicate-based glass, to which may optionally be added sodium borate, or based on aluminium phosphate and alumina powder, as described in U.S. Pat. Nos. 4,500,602 and 4,565,777. The object of these glasses is to lower the temperature from 1200.degree. to 1300.degree. C. to 800.degree. C. as from which the sealing of the cracks of the SiC coating becomes effective.
The same type of result is obtained by adding boron during the formation of the SiC coating by pack cementation (cf. U.S. Pat. No. 4,476,164). The glass which forms naturally on the SiC heated in air, e.g. during the reentry of space vehicles into the atmosphere, is a borosilicate with a lower melting point than silicon dioxide.
Finally, the sealing of the outer SiC coating by silicon dioxide and then by a glass based on silicate and borate is in particular described in EP-A-133 315.
Therefore all the above protections are based on the use of an outer SiC coating completed by silica glasses associated with B.sub.2 O.sub.3, Na.sub.2 O and P.sub.2 O.sub.5.
These protections operate correctly up to temperatures of approximately 1700.degree. C. at atmospheric pressure. However, under reduced pressure, the operating temperature of these materials is limited by the reaction of the silica (SiO.sub.2) on the silicon carbide corresponding to the following equation: EQU SiC+SiO.sub.2 .fwdarw.2 SiO+CO
Thus, the vapors of silicon monoxide and carbon monoxide produced perforate the viscous silica coating when the pressure produced exceeds that of the external atmosphere, namely:
2.9 KPa at 1500.degree. C. PA0 10 KPa at 1600.degree. C. PA0 32.6 KPa at 1700.degree. C. PA0 92.0 KPa at 1800.degree. C. PA0 0.72 KPa at 1700.degree. C., PA0 2.8 KPa at 1800.degree. C., PA0 9 KPa at 1900.degree. C., PA0 20 KPa at 2000.degree. C. PA0 (a) formation of a deformable porous substrate constituted by carbon fibers, PA0 (b) shaping the substrate, PA0 (c) densification of the shaped substrate for forming the matrix, PA0 (d) covering the outer surface of the matrix by an outer silicon carbide layer, PA0 (e) deposition of an intermediate coating not containing silicon serving as a reaction barrier between the silicon carbide and an oxide, PA0 (f) covering the intermediate layer by an external coating of an oxide chosen from among ThO.sub.2, ZrO.sub.2, HfO.sub.2, La.sub.2 O.sub.3, Y.sub.2 O.sub.3 and Al.sub.2 O.sub.3. PA0 (a) formation of a deformable porous substrate constituted by carbon fibers, PA0 (b) shaping the substrate, PA0 (c) densification of the shaped substrate for forming the matrix, PA0 (d) covering the outer surface of the matrix by an outer silicon carbide layer, PA0 (e) deposition of an intermediate coating serving as a reaction barrier between the silicon carbide and an oxide, said intermediate coating being chosen from among HfC, TaC, ZrC, W.sub.2 C, NbC, ThC.sub.2, ZrB.sub.2, HfB.sub.2, BN, HfN, ZrN, AlN, Pt, Ir, Os, Rh and Ru. PA0 (f) covering the outer layer by an oxide coating not containing silicon.
These conditions are those of silica alone.